1. Field of the Invention
The present invention relates to a resistance welding method, a resistance welding apparatus, and a method for manufacturing an electronic component using the resistance welding method and apparatus, and more particularly, to a resistance welding method of at least one lead wire of the electronic component.
2. Description of the Related Art
Some electronic components, such as a variable resistor, include lead wires disposed therein for being mounted on corresponding printed wiring boards. The lead wire is often bonded to an external electrode by resistance welding. The general structure of a resistance welding apparatus for the foregoing bonding is illustrated in FIG. 7.
The resistance welding apparatus includes a first welding electrode 1 clamping a lead wire 10, a second welding electrode 2, and a welding power source 3. The lead wire 10 is in pressure contact with a plate-like metal member 12 disposed on the upper surface of a main body 11 of the electronic component. With this arrangement, by passing a current through the second welding electrode 2 from the first welding electrode 1 via the lead wire 10 and the metal member 12, heat is generated at a contact portion between the lead wire 10 and the metal member 12, and thus, the generated heat causes the lead wire 10 and the metal member 12 to melt and to be bonded to each other.
Such a resistance welding method is disclosed in Japanese Unexamined Patent Application Publication No. 7-291344.
In the known method, a single second welding electrode 2 is provided. Since a current is likely to pass in a concentrated manner through a portion having a low resistance, a current flow path is close to the second welding electrode 2 as diagrammatically shown in FIG. 8. Hence, only a small portion of an end 10a of the lead wire 10 (a portion encircled by an ellipse 13 in FIG. 8) melts, which results in an insufficient bonding strength.
Also, since a concentrated current flows through a single portion, heat is excessively generated at the portion through which the concentrated current flows, which sometimes causes problems in that the plate-like metal member 12 is perforated and the end 10a of the lead wire 10 penetrates through the metal member 12. When the metal member 12 is perforated or the lead wire 10 penetrates through the metal member 12, an insufficient bonding strength is obtained.
In addition, since no device for measuring a bonding strength immediately after welding has been available, a defective product is sometimes forwarded to downstream steps. In order to prevent this problem, an additional inspection step is required, which increases the manufacturing costs.